FSTRANS(9)

NAME
fstrans, fstrans_setstate, fstrans_getstate, fstrans_start, fstrans_start_nowait, fstrans_done, fstrans_is_owner, fscow_establish, fscow_disestablish, fscow_run - file system suspension helper subsystem

SYNOPSIS
#include <sys/mount.h>
#include <sys/fstrans.h>
int
fstrans_setstate(struct mount *mp, enum fstrans_state new_state);
enum fstrans_state
fstrans_getstate(struct mount *mp);
void
fstrans_start(struct mount *mp, enum fstrans_lock_type lock_type);
int
fstrans_start_nowait(struct mount *mp, enum fstrans_lock_type lock_type);
void
fstrans_done(struct mount *mp);
int
fstrans_is_owner(struct mount *mp);
int
fscow_establish(struct mount *mp, int (*func)(void *, struct buf *, bool), void *cookie);
int
fscow_disestablish(struct mount *mp, int (*func)(void *, struct buf *, bool), void *cookie);
int
fscow_run(struct buf *bp, bool data_valid);

DESCRIPTION
The fstrans subsystem is a set of operations to assist file system suspension. These operations must not be used outside of file systems.

File systems supporting this subsystem must set the flag IMNT_HAS_TRANS in mnt_iflag.

File systems are always in one of these states:

FSTRANS_NORMAL
    Normal operations.
FSTRANS_SUSPENDING
    Preparing a suspension.
FSTRANS_SUSPENDED
    Suspended.

This state is represented by enum fstrans_state.

All file system operations use a fstrans lock. This lock is recursive. A thread already owning a lock will always get another lock. The lock has two variants:

FSTRANS_SHARED
    A lock that will be granted if the file system is in state FSTRANS_NORMAL.

FSTRANS_LAZY
    A lock that will be granted if the file system is in state FSTRANS_NORMAL or FSTRANS_SUSPENDING. It needs special care because operations using this variant will not block while the file system prepares suspension.

The lock variant is represented by enum fstrans_lock_type.

FUNCTIONS
The following functions comprise the API provided by fstrans.

fstrans_getstate(mp)
    Returns the current state of the file system mp.

fstrans_setstate(mp, new_state)
    Changes the state of the file system mp to new_state.

fstrans_start(mp, lock_type)
    Sets a lock of type lock_type on the file system mp.

fstrans_start_nowait(mp, lock_type)
    Like fstrans_start(), but will not wait for a state change of the file system when attempting to acquire the lock. The thread may still sleep while attempting to acquire the lock.

fstrans_done(mp)
    Releases a lock on the file system mp.

fstrans_is_owner(mp)
    Returns true if this thread is currently suspending the file system mp.

fscow_establish(mp, func, cookie)
    Establish a copy-on-write callback for the file system mp. The function func will be called for every buffer written through this file system.

fscow_disestablish(mp, func, cookie)
    Disestablish a copy-on-write callback registered with fscow_establish().

fscow_run(bp, data_valid)
    Run all copy-on-write callbacks established for the file system this buffer belongs to. If data_valid is true the buffer data has not yet been modified.

RETURN VALUES
The functions fstrans_setstate() and fstrans_start_nowait() return zero on success and an error value on failure.

EXAMPLES
The following is an example of a file system suspend operation.

    int
    xxx_suspendctl(struct mount *mp, int cmd)
    {
    	int error;
    	switch (cmd) {
    	case SUSPEND_SUSPEND:
    		error = fstrans_setstate(mp, FSTRANS_SUSPENDING);
    		if (error != 0)
    			return error;
    		/* Sync file system state to disk. */
    		return fstrans_setstate(mp, FSTRANS_SUSPENDED);
    	case SUSPEND_RESUME:
    		return fstrans_setstate(mp, FSTRANS_NORMAL);
    	default:
    		return EINVAL;
    	}
    }

This is an example of a file system operation.

    int
    xxx_create(void *v)
    {
    	struct vop_create_args *ap = v;
    	struct mount *mp = ap-a_dvp-v_mount;
    	int error;
    	if ((error = fstrans_start(mp, FSTRANS_SHARED)) != 0)
    		return error;
    	/* Actually create the node. */
    	fstrans_done(mp);
    	return 0;
    }

CODE REFERENCES
The actual code implementing this subsystem can be found in the file sys/kern/vfs_trans.c.

SEE ALSO
vfs_resume(9), vfs_suspend(9)

HISTORY
The fstrans subsystem appeared in NetBSD 5.0.

AUTHORS
The fstrans subsystem was written by Jürgen Hannken-Illjes <hannken@NetBSD.org>.